[LLVMdev] RFC: Replacing publicly accessible class IDs with getters

Matthew Curtis mcurtis at codeaurora.org
Wed Jan 30 10:02:47 PST 2013

Hello all,

In the process of porting the Polly plug-in to Windows we encountered a 
couple of issues stemming from the use (within Polly) of global data 
from LLVM.

By far the most common occurrence of this is the definition by a class 
of a publicly accessible static ID, the address of which is used to 
uniquely identify the class. For example

    class AliasAnalysis {

       static char ID; // Class identification, replacement for typeinfo


This turns out to be problematic on Windows for two reasons:

1) We found that Visual Studio actually defines two copies of the ID, 
one within the clang executable and another within the Polly library. 
This results in Polly being unable to identify LLVM passes. (This seems 
like a bug in Visual Studio, but we could not find a resolution other 
than changing LLVM as noted below).
2) We chose to use delay loading for symbols imported by Polly from 
clang. This allows the Polly dll to be loaded into any executable that 
provides the required symbols. However delay loading precludes the 
importing of data[1].

We would like to resolve these issues by replacing public access of the 
ID with a getter:

    class AliasAnalysis {

       static char ID; // Class identification, replacement for typeinfo

       static const void *getClassPassID();


Matthew Curtis

[1] http://msdn.microsoft.com/en-us/library/yx1x886y.aspx

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation

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